Different rolling methods are known, which allow to roll and calibrate metal products through a plurality of sequential passes.
For example, a hot rolling method is known for long products, which usually comprises four consecutive rolling passes to roll a metal product with a round section, and to obtain a rolled product always with a round section and a reduced diameter compared to the entering metal product.
The method provides a prior step of heating the metal product to a temperature comprised between 650° C. and 1250° C.
Subsequently, a first rolling pass is provided in which the entering metal product is deformed and rolled, to pass from an initial round section to an oval or elliptical section.
A second rolling pass is then provided in which the metal product is again deformed to obtain a round cross section shape, by means of a compression of the product in opposite directions with respect to the first rolling pass.
The first and second rolling pass induce on the product a great deformation, with a high percentage of reduction in section, which homogenizes the granular structure of the cross section from the center to the periphery.
The first and second rolling passes are made by a first rolling stand and a second rolling stand, each having two rolling rolls. The rolling rolls of the first rolling stand have their axes of rotation substantially orthogonal with respect to the axes of the rolling rolls of the second rolling stand.
Furthermore, the rolling rolls of the first rolling stand define between them a passage gap, with an oval shape, while the rolling rolls of the second rolling stand are provided with semi-circular throats to define a passage gap with a round shape.
A third and a fourth rolling pass are then provided, in which the product is rolled, progressively reducing its section and keeping it with a substantially round shape.
In particular, the fourth rolling pass obtains a calibration of the metal product so that it respects predefined tolerances of size and shape.
The third and fourth rolling passes are performed using a third and respectively a fourth rolling stand, each provided with three or four rolling rolls disposed angularly distanced from each other.
In particular, the rolling rolls of the third rolling stand have an offset angular disposition with respect to the rolling rolls of the fourth rolling stand.
One disadvantage of the solution described above is that obtaining a round cross section shape already in the second rolling pass penalizes the subsequent rolling steps downstream. In fact, to be able to conclude rolling in the fourth pass, and without further rolling passes, both the third and the fourth pass must have a rather narrow and arched channel between the rolling rolls. This entails an increase in the possibility of pinching the profile during the further rolling and section reduction, with a consequent deterioration in the quality of the rolled product.
DE 747 811 also discloses a rolling method in which five rolling passes are performed. In particular, the metal product is subjected to sequential rolling passes that modify progressively its cross section. The cross sections are, in sequence, a quadrangular-shaped cross section, a lozenge-shaped cross section, a quadrangular-shaped cross section, an elliptical-shaped cross section and, finally, a round-shaped cross section. This solution, however, is very expensive due to the high number of rolling passes performed, and provides a final product with imprecise final dimensions. In particular, the high deformations induced in the metal product, during the passage between the elliptical cross section and the round cross section, do not allow to obtain a calibrated final round product.
One purpose of the present invention is to perfect a rolling method for metal products that allows to obtain rolled products with excellent qualities of size and shape.
Another purpose of the present invention is to perfect a rolling method that allows to increase the mechanical resistance of the rolled product.
Another purpose of the present invention is to obtain a rolling method for rolled products that allows to increase the rolling action induced on the metal products.
Another purpose of the present invention is to perfect a rolling method for metal products that allows to extend the range of sizes of the metal products, starting from the same initial diameter. This means that, without changing the rolling rolls or the sizes of the passage gap, it is possible to calibrate several products, obtaining different exit diameters from the same entrance diameter.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.